Production of phosphate coatings on metals



Patented May 30, 1933 UNITED STATES PATENT OFFICE ROBERT R. TANNER AND HERMAN JOHAN LODEESEN, OF DETROIT, MICHIGAN, AS-

SIGNORS TO METAL FINISHING RESEARCH CORPORATION, OF DETROIT, MICHIGAN,

A CORPORATION OF MICHIGAN PRODUCTION OF PHOSPHATE COATINGS ON METALS No Drawing.

This invention has for its object to produce phosphate coatings on metals and more particularly to reduce the time within which a coating of a given thickness and composition may be produced upon a metal. More particularly the invention relates to the reduction in time necessary for producing a rust proof phosphate coating on surfaces of iron or steel. The details of the invention will appear as the description proceeds.

Various phosphate baths have been used to produce phosphate coatings on metal surfaces. In each case where success is achieved, a dilute bath is used in which the acid or acid phosphates attack the surface of the metal and form thereon integral phosphate coatings which are relatively insoluble. Such coatings will not be formed until the bath is within certain limits as to acidity and phosphates therein, etc., the proportions of the various ingredients varying considerably with different phosphates and with other modifying conditions, but being generally described by the term balance. In other words, if a phosphate bath will produce a coating upon a metal surface it is in balance so far as that metal is concerned. Otherwise, it is not in balance.

The operation of the balanced solution comprises reaction of the acid or phosphate in the solution with the surface of the metal and the production on the surface of the metal of relatively insoluble phosphates which are substantially integral with the surface. This action on the metal results in the release of hydrogen. The present invention, broadly speaking, consists in removing this hydrogen as it is released so that it does not form a hydrogen blanket on the surface of the metal. This removal of the hydrogen as it is released results in a material lessening of the time required for the production of a coating on metal, other conditions remain- 'ing the same.

Suificient removal of the hydrogen, as it is released, to materially expedite the coating operation, may be accomplished by the use 0 various oxidizing agents. Any oxidizing agent which, in the particular bath being used, will unite with the hydrogen as it is Serial No. 549,312.

released and reduce the visible bubbling action at the surface of the metal, is effective in reducing the processing time, but of course care must be taken to avoid the introduction into the bath of any material which will have a detrimental effect upon the coating.

In a bath containing ferrous phosphates, care must be exercised in selecting the oxidizing agent in order to avoid excessive action upon those phosphates. In such a bath, common oxidizing agents, including air, hydrogen peroxid, common manganates, permanganates, chromates and dischromates, and quinone, for example, act upon the ferrous phosphate in the bath and precipitate insoluble phosphates while releasing some free phosphoric acid. The result of the increased acidity of the bath is to increase the attack of the bath on the metal and increase, rather than decrease, the resultant bubbling and hydrogen blanket. Because of this, and of the disturbance in the balance of the bath, the use of such oxidizing agents in a bath containing ferrous phosphates tends to increase rather than decrease the processing time.

Manganese dioxide and hydrated manganic oxide act on ferrous iron in a rust proofing bath, but they also introduce manganese into the bath, which unites with some of the free phosphoric acid and forms manganese dihydrogen phosphate, a valuable constituent in a rust proofing bath. For this reason, these two oxidizing agents have some value, and manganese dioxide has been used commercially, but when introduced into a bath containing ferrous phosphate they do not reduce the hydrogen blanket and so are not useful for the purposes of the present invention.

For the above stated reasons, it is preferable, in a bath which contains ferrous phosphate, to use oxidizing agents which do not affect the ferrous iron, but which will react promptly with the hydrogen and so effectively reduce the hydrogen blanket. Examples of such oxidizing agents are the nitrates, nitrites and sulphites. A wide variety of these salts may be used, but in their use metals which will produce a harmful effect in the bath must be avoided. In a bath for treating iron, it is undesirable to introduce a salt of any metal below iron in the electromotive series, because the presence of such metal tends to decrease the rust resistant qualit of the resultant coating, and also has a ten ency to produce a loose and unsatisfactory coating unless the quantity of such metal and the conditions of its use are carefully regulated. For example some copper ma be allowed in the bath, with very marked e feet in reducing the processing time, but the amount of copper and acidity of the bath, etc., must be carefully regulated to produce a satisfactorily adherent coating, and at best the resultant coating is not as rust proof as the coating without the copper, unless there is immediate subsequent treatment. Therefore, generally speaking, the salts of metals not lower than iron in the electromotive series should be used in treating iron for a satisfactory rust proofing coat, and the salts of alkali metal and alkali earth metals have been found to be most desirable. For commercial purposes, sodium nitrate is readily available and has proved very satisfactory. Some nitrate present as an impurity in commercial sodium nitrate does not injure the result and seems to somewhat'accelerate the action.

Nitric acid may be introduced in small quantities as such, and has apparently an oxidizing efi'ect similar to that of the nitrates, but it increases the acidity of the bath and, therefore, tends to throw the bath out of balance. For that reason the salts are preferable to the acid.

Sodium tun state operates as an oxidizing agent for t e hydrogen without affecting the ferrous iron, and results in a very rapid coating operation, but produces a comparatively soft coating. On the other hand, while uranyl acetate does not expedite the action as much as sodium nitrate, the coating on iron, resulting from the use of uranyl acetate in the phosphate bath, is very hard and adherent and may be steel wooled to a high polish, but is comparatively thin. I

Uranyl acetate belongs to a class of oxidizing agents which have some effect upon ferrous men, but react therewith much more slowly than with the hydrogen, so that their reactlon in the bath is principally with the hydrogen and, while not as effective expediting agents for baths containing ferrous phosphate as oxidizing agents of the class previously referred to which do not affect the ferrous iron, they nevertheless do have some expediting effect in the bath since they tend to decrease rather than increase the hydrogen blanket. Other examples of such oxidizing agents are the common chlorates, bromates and iodates, and such organic compounds as methylene blue.

While a number of examples have been cited of oxidizing agents useful in all of the common phosphate baths for coating metal and examples in two other classes, one 0 which is less desirable when the bath contains ferrous phosphate and the other of which is detrimental for our present purposes in a bath containing ferrous phosphates, it will be understood that none of these lists is intended to be exhaustive, but merely by way of example.

The invention is particularly directed to producing relatively rust proof coatings upon surfaces of iron or steel within relatively short periods of time, but in its broader aspects the invention is also applicable to the production of phosphate coatings upon zinc, and also may be utilized to decrease the processing time in the production of phosphate coatings on iron or steel surfaces where copper is employed also as an expediting agent. One definite example of the application of this invention will be stated specifically, but with the foregoing explanation it will be understood that this specific statement is merely by way of example and that wide departures may be made therefrom within the principals set forth herein, and within the scope of the appended claims.

' The phosphate bath at present most widely used for producing rust proof coatings u on iron or steel surfaces is produced and rep enished by the use of dihydrogen phosphate powder produced in accordance with the patent of Green, Jones and Willard, No. 1,651,694 issued December 6, 1927 and a patent of Willard and Green No. 1,660,661 issued February 28, 1928. The action of a bath of this kind can be expedited so that reaction will be completed within from three to fourteen minutes, depending upon the character of the surface being processed and other variations, by the addition to the bath of about one half ounce of commercial sodium nitrate to one gallon of the bath. The expediting efl'ect of the sodium nitrate ma be realized to a greater or less extent with wide variation of amounts used, but that specified has been found to be a desirable roportion. The bath may be maintaine b adding ei ht pounds of sodium nitrate to eac two hun red forty two pounds of the phos phate powder used in replenishing the bath.

This maintains the bath in substantially the condition resulting from the addition of sodium nitrate as specified above.

The coating resulting from a bath to which sodium nitrate has been added is not perceptibly changed in composition by this addition, and its rust proofing effect is not materially altered. The beneficial effect is the cutting down of the processin time to a small fraction of that required y a similar bath without the addition of the oxidizing agent.

If other oxidizing agents than sodium nitrate are employed, there is a wide latitude in the quantity which may be used, but for best results it is preferred to employ a quantity of oxidizing agents which will be substantially equivalent in oxidizing effect to the quantity of sodium nitrate specified above.

The expediting effect of the oxidizing agent makes it practicable to employ the bath at a temperature lower than boiling, and this reduced temperature lessens the danger of coarse crystallization on the metal, known as sparkle, and this is another advantage of the invention in cases where sparkle is found to be troublesome.

Further details of the use of the solution at reduced temperatures are given in our application-Ser.No. 496,328, filed November.

When a replenishing powder is made by adding sodium nitrate or potassium nitrate to manganese phosphate, a slow reaction results which is undesirable. If any considerable time is to intervene between the mixing and the use of the powder, it is advisable to dust the sodium nitrate with some relatively inert material before mixing it with the phosphate. Finely powdered calcium carbonate is useful for this purpose. Five pounds of calcium carbonate thoroughly mixed with eight pounds of-sodium nitrate and this in turn mixed with two hundred forty two pounds of manganese phosphate makes a mixture which keeps for a considerable time without objectionable reaction, and calcium in this amount is not an objectionable ad dition to the constituents of the bath. Under some circumstances, zinc carbonate may be employed instead of calcium carbonate.

What is claimed is:

1. The method of expediting the coating, effect, upon a surface of metal of the group consisting of iron, zinc and their alloys, of a dilute solution containing phosphates of iron,

which consists in oxidizing hydrogen, as it is released in the coating operation at said surface, by use in the solution of an oxidizing agent having an oxidation potential not greater than the potential of the common bromates in a dilute phosphate solution.

2. The method of expediting the coating action upon a surface of ferrous metal of a dilute solution containing acid phosphate as the major portion of its coating chemicals, which consists in oxidizing hydrogen, as it is released in the coatingoperation at said surface, by use in the solution-of an oxidizing agent having an oxidation potential not greater than the potential of. the common bromates in a dilute phosphate solution.

3. The method of expediting the rustproofing of ferrous surfaces by a solution containing acid phosphate as the major portion of its coating chemicals, which consists in oxidizing hydrogen, as it is released in the coating operationat said surface, by use in the solution of a nitrate of a metal as high in the electromotive series as iron.

4. The method of expediting the rustproofing of ferrous surfaces by a solution containing acid phosphate as the major portion of its coating chemicals, which consists in oxidizing hydrogen, as it is released in the coating operation at said surface, by use in the solution of a nitrate of the group of metals consisting of alkali metals and alkaline earth metals.

5. A material for replenishing a coating bath, comprising water-soluble dihydrogen phosphate as the major portion of its coating chemicals and comprising also an oxidizing agent having an oxidation potential not greater than the potential of the common bromates in a dilute phosphate solution.

6. A material for rust-proofing, comprising powdered or granular dihydrogen phosphates and a nitrate of a metal as high as iron in the electromotive series, the phosphate constituting the major ingredient. V

7. A material for rust-proofing, comprising powdered or granular dihydrogen phosphate and sodium nitrate in the proportion approximately of 242 pounds of the phosphate and 8 pounds of nitrate.

8. A material for replenishing a phosphate bath, comprising powdered or granular dihydrogen phosphate of a metal from zinc to iron in the, electromotive series, sodium nitrate, and calcium carbonate coating the particles of nitrate, the phosphate being the 7 major ingredient.

9. The method of preparing a replenishing material for a phosphate bath, which consists in dusting and coating the particles of sodium nitrate with calcium carbonate and then mixing the dusted nitrate with a larger 

